Crystal chemical design, synthesis and characterisation of U(IV)-dominant betafite phases for actinide immobilisation

Crystal chemical design principles were applied to synthesise novel U4+ dominant and titanium excess betafite phases Ca1.15(5)U0.56(4)Zr0.17(2)Ti2.19(2)O7 and Ca1.10(4)U0.68(4)Zr0.15(3)Ti2.12(2)O7, in high yield (85–95 wt%), and ceramic density reaching 99% of theoretical. Substitution of Ti on the A-site of the pyrochlore structure, in excess of full B-site occupancy, enabled the radius ratio (rA/rB = 1.69) to be tuned into the pyrochlore stability field, approximately 1.48 ≲ rA/rB ≲ 1.78, in contrast to the archetype composition CaUTi2O7 (rA/rB = 1.75). U L3-edge XANES and U 4f7/2 and U 4f5/2 XPS data evidenced U4+ as the dominant speciation, consistent with the determined chemical compositions. The new betafite phases, and further analysis reported herein, point to a wider family of actinide betafite pyrochlores that could be stabilised by application of the underlying crystal chemical principle applied here.

. Naturally occurring minerals of the pyrochlore group have been shown to be stable under environmental conditions retaining actinides effectively over geological time periods, in excess of 1 billion years, much longer than the performance period of a geological disposal facility [11][12][13] .
Taking the simplified formula A 2 B 2 O 6 O' , the pyrochlore structure may be described as interpenetrating B 2 O 6 and (anti-cristobalite) A 2 O' networks, with corner sharing BO 6 octahedra and distorted AO 8 scalenohedra. The pyrochlore structure is related to the fluorite structure (comparable formula A 2 B 2 O 8 ), by ordering of both cations and oxygen vacancies, leading to a 2 × 2 × 2 superstructure, relative to the fluorite unit cell (a p = 2 a f ). The pyrochlore structure is stabilised, under ambient conditions, within the approximate radius ratio range: 1.46 ≲ r A /r B ≲ 1.78: below this threshold, a defect fluorite phase is stabilised, with cation and oxygen vacancy disorder; whereas, above the threshold, a monoclinic structure is stabilised, typified by La 2 Ti 2 O 7 8,14 . The archetype betafite CaUTi 2 O 7 is of specific interest as ceramic wasteform for actinide disposition, it is a component of the multiphase Synroc F wasteform and a ceramic phase assemblage designed to immobilise U-rich waste from 99 Tc production 4,[15][16][17][18][19] . There is a consensus that solid state synthesis of near single phase CaUTi 2 O 7 , with ≳ 95 wt% yield, is problematic [16][17][18][19] . Dickson et al. noted that CaUTi 2 O 7 "invariably coexisted with substantial portions of perovskite (CaTiO 3 ) and uraninite (UO 2 )" 16 ; and Vance et al. reported "several days failed to assure complete reaction… and the pyrochlore yields did not exceed ~ 75 wt%" 18 . These results are perhaps not altogether surprising given, that the radius ratio of CaUTi 2 O 7 , r A /r B = 1.75, is on the cusp of the pyrochlore stability field (herein, Shannon's effective ionic radii are employed 20 ). Interestingly, the radius ratio may be tuned into the stability field of the pyrochlore structure by oxidation of U 4+ to U 5+ /U 6+ , with coupled charge substitution, for example: Ca 1.4 U 0.7 Ti 2 O 7 , with U 4.5+ and r A /r B = 1.71 21 . However, U 4+ is the preferred speciation for wasteform applications, given the lower solubility and compatibility with reducing groundwaters, expected at depth, in a geological disposal facility. Vanderah et al. established that the pyrochlore structure may be stabilised for relatively large A-site cations, by substitution on the A-site of typical B-site cations, in excess of full B-site occupancy; remarkably, up to 25% substitution on the A-site may be tolerated 22 . We therefore applied this crystal chemical design principle to hypothesise novel U 4+ dominant, and titanium excess, betafite compositions with radius ratio, r A /r B = 1.69, within the pyrochlore stability field, nominally Ca 1.00 U 0.50 Zr 0. 20 18 . The novel betafite phases designed and reported herein, point to a wider family of actinide pyrochlores that could be stabilised by application of the same crystal chemical principle, which we hope will be more extensively investigated.

Results and discussion
Powder X-ray diffraction (PXRD) analysis of the synthesised products demonstrated the formation of pyrochlore structured compounds (space group Fd 3 m) with the presence of only minor or trace secondary phases (Fig. 1) Fig. 1d. No free uranium oxides were detected in the XRD of any product, which, together with the well sintered microstructures (see below), suggested that the solid state reactions were not kinetically hindered.
The microstructures of sintered betafite ceramics are shown in Fig. 2, and were fully consistent with the phase assemblage determined from XRD data. The Energy Dispersive X-ray (EDX) determined compositions of constituent phases are presented in Table 1 and Table S1 (supplementary material); EDX spectra are presented in Figs. S1-S4.
Nominal composition Ca 1.00 U 0.50 Zr 0.20 Ti 2.30 O 7 exhibited a dense microstructure, with little porosity observed (Fig. 2a). From greyscale contrast, it was evident that the microstructure comprised three distinct phases. The major phase (labelled B) was identified as betafite, as determined by the coincidence of U, Ca and Ti signals in EDX spectra (Fig. S1). The presence of the Zr Lα emission line at ca. 2 keV was indicative of the solid solution of Zr in the betafite phase of all products. Minor phases were determined to be TiO 2 and CaTiO 3 (labelled R and P, respectively; EDX spectra presented in Fig. S1). Nominal composition Ca 0.96 U 0.72 Zr 0.17 Ti 2.15 O 7 also presented a dense microstructure, Fig. 2b, that comprised a majority betafite phase with minor TiO 2 and UTi 2 O 6 (labelled  Br; EDX spectra presented in Fig. S2). The addition of 10wt% Fe was found to have a significant impact on the phase assemblage and microstructure of nominal composition Ca 0.96 U 0.72 Zr 0.17 Ti 2.15 O 7 . As shown in Fig. 2c, in addition to a major betafite phase, minor CaTiO 3 and Fe 2 TiO 4 (ulvospinel, labelled U) were observed, together with considerable porosity (EDX spectra presented in Fig. S3). 10wt% Ni addition to nominal composition Ca 0.96 U 0.72 Zr 0.17 Ti 2.15 O 7 also produced a dense microstructure, Fig. 2d, that comprised a majority betafite phase with minor TiO 2 and UTi 2 O 6 , and residual Ni metal (EDX spectra presented in Fig. S4). The observed size of the Ni phase (see inset to Fig. 2d) was consistent with that of the starting Ni metal reagent. This, and the absence of any Ni Kα emission line in the EDX spectra of Fig. S4a-c, demonstrated no detectable reaction of the Ni metal had occurred.
The EDX chemical compositions of the major betafite phases were close to those targeted and evidenced an excess of B-site cations within precision: Ca 1 Table 1). The EDX determined compositions implied average uranium oxidation states of 4.04 + and 4.00 +, respectively, assuming Ti 4+ speciation (note: synthesis conditions were not considered sufficiently reducing to afford significant reduction to Ti 3+ ).  Fig. 2 and discussed above. The average bulk uranium oxidation state in the betafite ceramics was investigated by analysis of X-ray absorption near edge structure (XANES) at the U L 3 -edge; data are presented in Fig. 3. The oxidation state was determined by the linear regression method as has been previously proposed 23,24 , using the edge position, E 0 , of reference compounds of known oxidation state to establish a calibration line, as shown in Fig. 4. The bulk average uranium oxidation states determined by this method evidenced the presence of only U 4+ in the betafite ceramics, within experimental error, as shown in Table 3). The difference in features of the white line maximum and near-edge structure of the U L 3 -XANES, for compounds with the same nominal oxidation state shown in Fig. 3, reflect sensitivity to the specific local environment of the U absorber in the reference compounds.
XANES data were also analysed by combinatorial linear combination fitting (LCF) 25,26 , using the library of reference compounds to estimate the fraction of contributing oxidation states. Significance tests of the goodness of fit R-factor were undertaken using the Hamilton R-factor ratio test, with a significance level of α = 0.05 27 . From these fits the weighted mean oxidation state and associated root mean square error approximation were calculated. The plots of best fit are shown in Fig. S5. The results of the combinatorial LCF, summarised in Table 3, also evidenced a dominant average bulk uranium oxidation state of U 4+ , but with a minor U 5+ contribution; no significant U 6+ contribution was determined.
The bulk average oxidation state of uranium in nominal Ca 1.00 U 0.50 Zr 0.20 Ti 2.30 O 7 and Ca 0.96 U 0.72 Zr 0.17 Ti 2.15 O 7 was further investigated using X-ray Photoelectron Spectroscopy (XPS). As shown in Fig. 5A, the spectra of both compositions presented two main peaks, U 4f 7/2 and U 4f 5/2 (separated by ca. 11.0 eV, due to spin-orbit splitting) and two satellite peaks, Sat. 7/2 and Sat. 5/2 . Fitting of Sat. 5/2 was used to assess the contributing uranium oxidation states; for both compositions, the Sat. 5/2 peak could be fitted by a majority U 4+ contribution with a minor U 5+ contribution, as shown in Fig. 5B; no contribution from U 6+ was apparent. Similarly, deconvolution of the U 4f 7/2 peak for nominal Ca 1.00 U 0.50 Zr 0.20 Ti 2.30 O 7 and Ca 0.96 U 0.72 Zr 0.17 Ti 2.15 O 7 evidenced a majority contribution from U 4+ and a minor contribution from U 5+ , as shown in Fig. 6; no U 6+ contribution was evidenced. The positions of two Table 2. Phase assemblage derived from Rietveld analysis of PXRD data; EDX compositions of betafite and secondary phases are reported, respectively, in Table 1 and Table S1.  was marginally more oxidised (U 4+ 88%, U 5+ 12%). Overall, the average bulk oxidation states determined from U L 3 -edge XANES and U 4f 7/2 XPS are in good agreement, and evidence dominant U 4+ speciation with a minor U 5+ contribution of around 10%. These bulk analyses are consistent with the dominant U 4+ oxidation state inferred from EDX analyses of the betafite phase, within which the uranium is overwhelmingly partitioned. Our estimation of the U 5+ content determined from XPS is based on curve fitting, assuming intrinsically symmetrical U 4f 7/2 lines. In reality these lines are slightly asymmetrical (multiplets) and the Shirley background is also only an approximation. These assumptions are expected to result in a small overestimation of the U 5+ content. We can therefore conclude that the uranium speciation in the betafite phases is primarily U 4+ , with a U 5+ contribution of no more than 10%.
Subsequent to this study, Blackburn et al. investigated the zirconolite solid solution CaZr 1-x Th x Ti 2 O 7 , and discovered the formation of a new betafite phase, for x > 0.4; a single phase was produced for x = 0.6, with a determined composition of Ca 1.00(2) Zr 0.33(2) Th 0.54(1) Ti 2.13(2) O 7 29 . The radius ratio of this phase is r A / r B = 1.70, within the pyrochlore stability field, and identical to that of the betafite phases designed and reported here. In contrast, McCauley and Hummel, reported synthesis of the end member composition CaThTi 2 O 7 , to be unsuccessful 30 . Indeed, this is consistent with a radius ratio, r A /r B = 1.79, outside of the pyrochlore stability field. Therefore, Ca 1.00(2) Zr 0.33(2) Th 0.54(1) Ti 2.13(2) O 7 may also be considered an example of a pyrochlore structure stabilised by Ti excess on the B-site and partial occupancy of the A-site. This example, and those reported herein, point to a wider family of actinide pyrochlores that could be stabilised by application of the underlying crystal chemical principle applied here.

Conclusion
Novel U 4+ dominant and titanium excess betafite phases, Ca 1.15(5) U 0.56(4) Zr 0.17(2) Ti 2.19(2) O 7 and Ca 1.10(4) U 0.68(4) Zr 0.15(3) Ti 2.12(2) O 7 , were successfully synthesised in high yield (85 -95 wt%), by application of the crystal chemical design principle of targeting excess B-site cations to the A-site in the pyrochlore structure. This design strategy enabled the radius ratio to be effectively tuned into the pyrochlore stability field, and the synthesis of U 4+ betafite ceramics in high yield, and with high relative density (> 99% theoretical), for the first time. U L 3 -edge XANES and U 4f 7/2 and U 4f 5/2 XPS data evidenced U 4+ as the dominant speciation, consistent with EDX determined compositions. Reconsideration of the recently reported thorium betafite phase, Ca 1.00(2) Zr 0.33(2) Th 0.54(1) Ti 2.13(2) O 7 established that this compound is also effectively stabilised by the same crystal chemical design principle applied here. More broadly, this example, and the novel betafite phases designed and reported herein, point to a wider family of actinide pyrochlores that could be stabilised by application of same crystal chemical principle. The observed incorporation of Fe within Ca 0.90(5) U 0.71(5) Zr 0.15(2) Ti 1.97(3) Fe 0.28(6) O 7 demonstrates a further degree of chemical flexibility which could be exploited in terms of this crystal chemical design principle, with partial Ti 4+ occupancy of the pyrochlore A-site facilitated by co-substitution on the B-site of a suitable cation.

Methods
Caution. Uranium is an alpha emitter. Manipulations, synthesis and characterisation were performed in a materials radiochemistry laboratory in a controlled area, using HEPA filtered fume hoods and a dedicated glove box, following risk assessments and monitoring procedures 31 . www.nature.com/scientificreports/ Betafite ceramics were produced by solid-state reaction -sintering between stoichiometric quantities of CaTiO 3 (Sigma-Aldrich, purity ≥ 99% trace metals basis), ZrO 2 (Sigma-Aldrich, purity ≥ 99%) and TiO 2 (Sigma-Aldrich, purity ≥ 99%) and UO 2 (purity > 99%). UO 2 with a small particle size of 1 μm was selected as a reagent, given previous suggestion that pyrochlore synthesis may be kinetically hindered by the use of UO 2 16,18 O 7 precursor was divided into three parts; 10 wt% of metallic Fe (Acros Organics, purity ≥ 99%) or Ni (Acros Organics, purity ≥ 99.9%) was added to one part of the precursor, by mixing in a mortar and pestle. These compositions were fabricated to investigate the potential for metallic Fe and Ni to maintain U 4+ by scavenging trace oxygen. Batched material was uniaxially pressed in a 10 mm steel die under uniaxial pressure of 180 MPa and sintered at 1320 °C for 2 h, with a ramp rate of 5 °C·min −1 , in flowing high purity nitrogen (250 mL·min −1 ).
The density of the sintered ceramics was measured based on Archimedes displacement method. For phase analysis, sintered ceramics were sectioned using a diamond saw and a small segment was ground to a fine powder in a mortar and pestle. Examination of the phase assemblage was performed by powder X-ray diffraction (PXRD; D2 Phaser, Bruker, Karlsruhe, Germany) with a Cu K α source, Ni K β filter operating voltage of 30 kV and current of 10 mA. Quantitative phase analysis (QPA) was performed by Rietveld refinement using the GSAS software package and the ExpGUI interface 32 . The microstructure of sintered pellets was examined by Scanning Electron Microscopy (SEM) in backscattered electron mode using a Hitachi TM3030 microscope coupled with a Bruker Quantax 70 EDX system. Samples were prepared for analysis by polishing sections of ceramic to a 0.25 µm finish using SiC paper and progressively finer diamond pastes. Semi-quantitative compositions were acquired by Energy Dispersive X-ray spectroscopy (EDX) based on 10 EDX data points; a stoichiometry of 7 O atoms per formula unit was assumed, given the low accuracy of EDX to light element determination.
Average uranium oxidation states were determined by analysis of U L 3 -edge X-ray absorption near edge structure (XANES). The ceramic products and reference compounds for XANES measurement were prepared by homogenously mixing powder specimens with polyethylene glycol and uniaxially pressing to form 13 mm diameter pellets of approximately one absorption length. XANES data were acquired on Beamline B18 at Diamond Light Source (DLS; Oxford, UK). The beamline configuration comprised a water cooled vertically collimated Si mirror, a double crystal Si(111) monochromator, a double toroidal focusing mirror, and harmonic rejection mirrors. Uranium L 3 -edge XANES spectra were recorded in transmission mode between 17,000 and 17,410 eV. To improve data quality, the beam spot size was defocused to ca. 1.0 mm and multiple scans were acquired and averaged. Data reduction and linear combination fitting were performed using the Athena software package 25 .
X-ray photoelectron spectroscopy (XPS) data of uranium were recorded at room temperature using a SPECS Phoibos 150 hemispherical analyser, using monochromated X-rays (SPECS, microfocus source, 15 kV, 50 W, spot size: 0.3 mm). Samples were glued with 2-component epoxy glue (Dynaloy 325) and stored under vacuum at room temperature for 7 days to allow cure finishing and avoid surface oxidation. Samples were scraped by a diamond file under vacuum (1 × 10 −7 mbar) to produce bulk representative surfaces. The energy scale for XPS was calibrated with the Au 4f 7/2 (84.0 eV) and Cu 2p 3/2 (932.7 eV) emissions. The vacuum in the photoemission chamber were 1.2 × 10 −10 mbar. Charge compensation was performed by a flood gun (1 eV, 15 mA). The obtained spectra were deconvoluted using Gaussian function and the baseline subtracted with a Shirley function. The location and full-width at half-maximum (FWHM) of the components were allowed to vary freely, but the width of the components was set to be equal in each fit.

Data availability
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.